1. Field of the Invention
The present invention relates to disk drives and more particularly to a technique for repeatable runout compensation for a track on a disk of a disk drive.
2. Description of the Prior Art
In order to remain competitive, disk drive manufacturers must continually provide increased storage capacity while rigidly controlling manufacturing cost. One key to increased storage capacity is increasing track pitch. A critical limiting aspect of increased track pitch is the ability of the disk drive servo system to maintain a read/write head location within a budgeted distance from a track centerline to avoid corrupting data on adjacent tracks during a write operation. This distance is sometimes known as xe2x80x9cTrack Misregistrationxe2x80x9d or TMR. One influence on the TMR budget is the effect of written in runout during servowriting. During operation, the disk drive attempts to follow the path of a track on the disk defined by the position of embedded servo sectors laid down during servowriting. The embedded servo sector positions can deviate from a so-called perfect circle due to, for example, vibrations which occur during the servowriting process. Such vibration causes the track to thenceforth exhibit a track-dependent repeatable runout (RRO) which appears to the servo system as a position error signal (PES) for each servo sector in the track. The error can be xe2x80x9clearnedxe2x80x9d and compensated for over a relatively lengthy process which scales with the number of tracks and the number of iterations for learning. One method of learning RRO is disclosed in commonly assigned U.S. patent application Ser. No. 08/946,805 entitled xe2x80x9cREPEATABLE RUN OUT CANCELLATION IN SECTORED SERVO DISK DRIVE POSITIONING SYSTEM,xe2x80x9d the entire disclosure of which is hereby incorporated by reference as if fully set forth herein.
Despite the apparent advantages of compensating for RRO, present practice in the industry simply factors in a statistical budget for the error and scraps tracks which appear to have deviation beyond what the budget allows. This is driven by the economics of volume manufacturing, which dictates that increased time and capital equipment required will not support the benefit from RRO learning. Nevertheless, the demand for capacity continues to challenge servo system designers as the track pitch of disk drives grows inexorably smaller.
Accordingly, there exists a need for a disk drive having improved RRO compensation without significantly decreasing manufacturing throughput or unnecessarily increasing manufacturing costs.
The present invention may be embodied in a method, and related apparatus, for determining a repeatable runout (RRO) compensation value set for a track on a disk in a disk drive, wherein the disk drive is initially manufactured and then shipped for installation in a host system. The track comprises a plurality of embedded servo sectors that define a circumferential path having RRO which requires compensation during track following. The method comprises determining a first RRO compensation value set for the track to a first desired learning level by measuring the RRO for the track based on a first predetermined number of disk rotations during the initial manufacturing of the disk drive. Then, after the disk drive is shipped for installation in the host system, determining a second RRO compensation value set for the track to a second desired learning level by measuring the RRO for the track based on at least a second predetermined number of disk rotations, wherein the second predetermined number of disk rotations is greater than the first predetermined number of disk rotations.
Further, the disk drive may include read/writeable non-volatile semiconductor memory and the RRO compensation value sets may be stored in the read/writeable non-volatile semiconductor memory. The read/writeable non-volatile semiconductor memory may be an electrically erasable programmable read only memory or a FLASH memory. Alternatively, the RRO compensation value sets for the track are stored in the servo sectors and/or in a portion of data sectors between the servo sectors.
Also, the first predetermined number of disk rotations may be within the range from 4 to 8 disk rotations, and the second predetermined number of disk rotations may be at least 100 disk rotations. Alternatively, the first predetermined number of disk rotations may be less than 20 disk rotations, and the second predetermined number of disk rotations may be at least 20 disk rotations. Additionally, the second RRO compensation value set may be a running average of the measured RRO for the track
An alternative embodiment of the invention may reside in a method, and related apparatus, for determining a plurality of RRO compensation value sets for a plurality of tracks on a disk in a disk drive wherein the disk drive is initially manufactured and then shipped for installation in a host system. Each track comprises a plurality of embedded servo sectors that define a circumferential path having RRO which may require compensation during track following. The method comprises determining, during the initial manufacturing of the disk drive, first RRO value sets for a first portion of the tracks to a first desired learning level by measuring the RRO for the first portion tracks based on a first predetermined number of disk rotations, and first RRO value sets for a second portion of the tracks to a second desired learning level by measuring the RRO for the second portion tracks based on a second predetermined number of disk rotations. Then, after the disk drive is shipped for installation in the host system, determining second RRO value sets for the second portion tracks to a third desired learning level by measuring the RRO for the second portion tracks based on at least a third predetermined number of disk rotations, wherein the third predetermined number of disk rotations is greater than the second predetermined number of disk rotations.
Another alternative embodiment of the invention may reside in a method, and related apparatus, for determining a plurality of RRO compensation value sets for a plurality of tracks on a disk in a disk drive wherein the disk drive is initially manufactured and then shipped for installation in a host system. Each track comprises a plurality of embedded servo sectors that define a circumferential path having RRO which may require compensation during track following. The method comprises determining, during the initial manufacturing of the disk drive, first RRO value sets for a first portion of the tracks to a first desired learning level by measuring the RRO for the first portion tracks based on a first predetermined number of disk rotations. Then, after the disk drive is shipped for installation in the host system, determining RRO value sets for a second portion of the tracks to a second desired learning level by measuring the RRO for the second portion tracks based on a second predetermined number of disk rotations.